Problems Aerospace Still Has To Solve

Aerospace research is a long-term business. Moving from laboratory to production can take 20 years—longer if the result is as significant as a new airframe configuration or propulsion architecture. So industry has already chosen its path to the mid-2020s and has few roads left to take to the mid-2030s. Beyond that, the route is less certain, but even now investment decisions are being taken that begin to map those roads.

. . . CLEANER

Not everyone believes in climate change. Not everyone believes in electric propulsion. But the world’s airlines have committed to halting, then reversing the growth in their carbon footprint to prevent their environmental impact constraining the future expansion of air travel. That undertaking is driving the aeronautical research now shaping the future of aviation.

Lower-drag aerodynamics, lighter structures, more efficient engines, electrification of systems, streamlined operations, even unconventional configurations and propulsion are all avenues being pursued as industry strives to maintain, and if possible accelerate, its historical pace of efficiency improvement to keep up with, and enable, projected air traffic growth.

Since the first jets, airliner efficiency has improved on average by 1-2% a year. To achieve the long-term goals set as targets for research underway in Europe and the U.S., this rate of reduction in specific fuel consumption (sfc) and carbon dioxide emissions must be maintained or increased.

But the historical trend has come in generational steps. The AirbusA320neo in 2016 has 15% lower fuel burn than the original A320 in 1988 and will reach 20% by 2020 with further enhancements, but this is less of an improvement than the downward slope suggests.

Most of the efficiency enhancement in the A320neo and Boeing 737 MAX, and even the A350 and 787, comes from new, higher-bypass turbofans, and the engine manufacturers have mapped out plans to deliver 1%-per-year improvements into the 2020s. But more will be needed to reach the goals.

And each new step does not come easy. It cost Pratt & Whitney $10 billion over 20 years to develop the PW1000G geared turbofan and deliver an initial 15% fuel-burn reduction over the V2500 on the A320. The company has a road map to a further 10-15% in savings by the mid-2020s.

But NASA research suggests fuel burn could be reduced by up to 50% by 2025, and 60% by 2030, if a suite of technologies beyond just the engines is brought to bear. This is why researchers looking long term are working on new aircraft configurations and propulsion architectures.

NASA’s research indicates today’s tube-and-wing design could deliver fuel savings up to 45% with advanced aerodynamics, structures and geared turbofans, but to go much further would require new thinking. Ultra-efficient configurations being studied include the hybrid wing-body, truss-braced wing, and designs with lifting fuselages, embedded engines and boundary-layer ingestion for lower drag.

A conservative guess is that the tube-and-wing will endure into the 2030s thanks to drag reductions from active flutter suppression to enable slender, flexible wings, and natural and hybrid laminar flow enabled by aerodynamic advances. But upswept gull wings or overwing nacelles will be needed to accommodate large-diameter, ultra-high-bypass engines.

Electric propulsion is beginning small, with two-seat trainers, but by 2030 hybrid-electric architectures may be ready to power aircraft with fewer than 100 seats—or so Airbus and NASA believe. Going larger than 100 seats will be more difficult, and the key to further efficiency and emissions improvements will likely lie with distributed propulsion, where tighter integration of airframe and engines enables aerodynamic advances beyond that possible by other means.

Eventually, aviation will have to wean itself off fossil fuels or risk being the last user of a dwindling resource. Synthetic kerosenes from sustainable sources will extend the life of liquid fuels by decades, but ultimately aviation will need another energy source if it is to continue reducing its carbon footprint. The hope is batteries will advance to meet aviation’s needs, or other means of high-density energy storage will emerge that ensure a clean future.

. . . QUIETER

Aircraft are not quiet neighbors, which is why building a new airport is nigh on impossible, adding or extending a runway takes decades and preserving a downtown heliport in a clamorous city is a hard fight. Add in the potential for supersonic airliners, unmanned aircraft and air taxis, and noise becomes one of the biggest challenges to aviation’s future.

Understanding and mitigating the noise produced by open rotors is critical if the fuel-efficient engines are to power airliners. Credit: NASA

With the world’s major population centers projected to become sprawling megacities, airports will come under increasing geographic pressure even as demand for travel grows. High-speed rail or Hyperloop tubes could displace aviation for short-haul travel unless airports can increase their capacity without worsening their impact on the surrounding neighborhoods.

So far, demand for greater flight efficiency has aligned with the need for lower airport noise—higher bypass ratios reducing both fuel consumption and sound generation—but they could be approaching a divergence. Ever-higher bypass ratios for more efficiency mean slimmer nacelles with less area for sound attenuation, or no nacelles at all in the case of open rotors.

While NASA research suggest the traditional tube-and-wing airliner configuration still has significant potential for fuel-burn reduction, the same is not true for noise. Engines may become quieter as fans get bigger, but if they remain hanging under the wing there is a limit to their quietness. The best NASA expects from a tube-and-wing design in the next 10-15 years is a cumulative 20-30 dB below Stage 4 limits.

Keeping aircraft noise well within airport boundaries and away from neighbors will require reductions of at least 40-50 dB. In addition to the mitigation of airframe noise sources such as landing gear, slats and flats—perhaps through smooth, morphing high-lift systems—engines may have to be shielded from the ground. This will require aircraft configurations different from those flying today.

The tube-and-wing design could be modified to locate the engines where the wing and fuselage provide some shielding of fan or jet noise. NASA studies suggest the overwing nacelle and mid-fuselage nacelle configurations could reduce noise by 30-40 dB. But the hybrid wing-body with its broad fuselage offers the highest shielding and reductions of 40-50 dB. Such shielding configurations could be essential to making fuel-efficient open rotors acceptable.

Making helicopters quieter is difficult because of the fundamentals of rotors, but progress is being made, both in the design and operation of rotorcraft. The latest generation of helicopters is quieter, and technologies and procedures are in development that will reduce sound levels 10 dB and noise footprints 50% by 2020. But more progress is needed if heliports are to be preserved, let alone expanded, and rotorcraft—and vertical-takeoff-and-landing (VTOL) aircraft in general—are to play a significant role in inter- and intra-city transport.

But airliners and helicopters will not be the only airborne sources of noise nuisance in the near future. The expected explosion in the use of unmanned aircraft systems (UAS), particularly for package delivery, will come with significant acoustic baggage. Today’s multicopter VTOL small UAS, or drones, are known to be annoying. Research to characterize the noise, establish limits and reduce the impact is in its infancy but will be critical not only for UAS, but also for ambitions to break the gridlock on the ground with Uber-style on-demand aviation using new types of aircraft.

. . . FASTER

Globally, the pace of communication and commerce has accelerated with each revolution in information technology, but air travel is no faster—in fact is slower—than at the beginning of the jet age. Will this growing disparity in speeds between life and flight mean the return of the supersonic transport (SST)?

Many in industry, from Boeing executives to startup entrepreneurs, believe the SST will return, but the question is when. For newcomers Aerion and Boom Technology, the time is now. For NASA, Boeing, Gulfstream and Lockheed Martin, the time will come when the sonic boom has been eliminated as a barrier to supersonic flight over land. For others, supersonic is too slow, and hypersonic is the answer.

The impediments to supersonic air travel are environmental and economic­—boom, noise, emissions, fuel-burn and cost. NASA abandoned efforts to develop a 300-seat, Mach 2.4, transoceanic SST in 1999 when industry said passengers would not pay a premium for speed. Research resumed in 2006, but has focused on reducing sonic boom to a level that will enable supersonic flight over land, now viewed as essential to the economic viability of high-speed transport.

Removing the ban on supersonic flight over land requires regulatory action, and the regulators require data before acting. So NASA’s plan, supported by Boeing and Lockheed Martin, is to fly a low-boom demonstrator in 2019 to gather data on community response to sonic booms reduced from double bangs to soft thumps by airframe shaping. That data will be provided to the FAA and International Civil Aviation Organization in the early 2020s to support lifting the ban on supersonic overland flight.

The single-engine Quiet Supersonic Transport X-plane will mimic the shockwave signature of a larger, 80-100-seat, Mach 1.6-1.8 airliner—slower than the Concorde, but with a boom level of 75 PNLdB compared with 105 PNLdB for the 1960s design. With advances in aerodynamics and engines, the new aircraft would also burn about a third less fuel than Concorde, making it more economically attractive.

Rotorcraft are poised to fly faster and farther, potentially led by designs such as Bell’s 280-kt. V-280 tilt rotor. Credit: Bell Helicopter

But some advocates of speed do not want to wait until the mid-2020s for regulations to change and low-boom technology to mature. Aerion hopes to launch development of its 12-passenger, Mach 1.5 business jet by mid-2016, supported by Airbus Group, with deliveries to begin by 2023. Boom Technology plans to fly a subscale demonstrator for its 40-seat, Mach 2.2 airliner by the end of 2017. Boom says its SST will be able to compete economically with subsonic business-class travel.

Neither design is low-boom. Both startups are relying on improved efficiency to make their aircraft economically viable, flying supersonically over water and subsonically over land—at least until the rules are changed. They will be first to test the premise that a market for supersonic travel does exist, at least among the wealthy. Aerion has an order from fractional-ownership operator Flexjet and Boom has a commitment from Virgin Galactic. High-speed flight for the masses is likely still decades away.

Travel at speeds beyond supersonic remains an ambition, but hypersonic aircraft for air transport or space access will only come once the military has matured the technology. After decades of on-off research, and recent rapid advances by China and Russia, the U.S. looks set to develop its first hypersonic weapons. Once scramjet engines move from demonstration to production, research into turbine- or rocket-based combined-cycle propulsion will pick up and routine hypersonic flight will be a step nearer.

. . . CLOSER

Most of the world’s cities have grown beyond the capacity of their ground transportation infrastructure, and building new highways and railways in built-up areas is expensive and disruptive. Aviation visionaries see this as an opportunity to bring a third dimension to urban travel.

NASA’s vision is autonomous electric aircraft flying commuters over congested highways to heliports close to home and work.

They have been called flying cars, personal air vehicles and air taxis. The latest term is on-demand mobility, but the idea is as old as aviation—using aircraft to escape the limitations of ground transport, now mostly in the form of roads congested with commuters.

Advocates see a convergence of technologies being developed for unmanned aircraft, primarily autonomous systems and electric propulsion, with the critical need for an alternative, or adjunct, to building more road and rail links. But they acknowledge the barriers to feasibility are many, and include safety, certification, operating cost, ease of use, airspace integration, noise and emissions.

Many of those hurdles will be tackled first by unmanned aircraft, including the certification of small UAS, their safe integration into low-altitude airspace and acceptability of aircraft noise within the community. Experience over the next few years, particularly with establishing routine urban delivery services using VTOL UAS, will play a key role in determining the feasibility of Uber-style on-demand aviation.

. . . CHEAPER

Aerospace has acquired a reputation for troubled programs—step-change aircraft such as Boeing’s 787 and Lockheed Martin’s F-35 have proved far more costly and complex to develop than expected. As commercial manufacturers strive for greater efficiency and military developers for leaps in capability, affordability is a looming concern for both.

Cost and risk will play a central role in defining the capabilities and technologies in the next generation of U.S. steal fighters. Credit: Northrop Grumman

It did not seem to matter as much in the heydays of the 1950s and ’60s, when programs with technically unachievable goals were launched and canceled with astonishing regularity. But today shareholders and taxpayers, as well as customers, demand accountability, and program performance has become a crucial issue.

Development of more advanced and integrated design tools is helping, including the maturing of model-based systems engineering that more accurately predicts the interactions that can cause problems during development. Greater use of computational analysis and optimization early in design is allowing more options to be explored in search of the best solution to the requirements. Increasing automation in engineering and manufacturing is bringing down cycle times and cost.

But at the same time, technology advances ranging from materials to manufacturing are enabling design permutations that risk making the engineers’ task more complex. Additive manufacturing is enabling the 3-D printing of multifunction parts that once were impossible to design or make. Countering that trend are the long-sought, hard-learned lessons from problems with recent programs that requirements must be traded, defined and locked in. New technologies must buy their way on to the platform. The trend is already visible in the U.S. Air Force’s restrained Northrop Grumman B-21 bomber and the reengined A320neo and 737 MAX.

But it will be the next all-new generation, of combat aircraft or commercial airliners, that will show whether the lessons of the 787 and F-35 really have been learned. The Air Force is already talking down the need for an “exquisite” next-generation platform to ensure air dominance beyond 2030. Airbus and Boeing have recognized that each new generation of airliner cannot keep costing more than the last. And the next all-new fighter or single-aisle airliner may not come before 2030, by which time a great deal of advanced technology will have been developed and be looking for a home.

This article was first published on May 6, 2016 as part of Aviation Week & Space Technology’s 100th anniversary issue.